Laser absorption spectroscopy and in particular diode-laser absorption spectroscopy with the use of a tunable laser, also referred to as TDLAS (tunable diode laser absorption spectroscopy), is particularly well suited to determining at least one chemical and/or physical parameter of a gaseous measurement medium.
By way of laser absorption spectroscopy it is possible to very precisely determine the concentration or the content of a gas that is present in a measurement environment or in a measurement medium. Furthermore, it is also possible to determine further parameters, for example temperature or pressure.
With TDLAS, the measurement medium is irradiated by radiation of a tunable laser. In this technique, a wavelength of the radiation is periodically modulated in a predetermined wavelength range, wherein the wavelength range passed through by the laser can include one or several absorption bands of the gas to be analysed. The wavelength range covered is determined by the laser used, and for example more precisely, by the diode laser used. A multitude of lasers and diode lasers are known. So-called DFB lasers (distributed feedback lasers) can cover wavelength ranges between approximately 700 nm and approximately 3 μm. So-called VCSEL lasers (vertical-cavity surface-emitting lasers) can cover wavelength ranges up to approximately 2.1 μm, and QCL lasers (quantum cascade lasers) can cover wavelength ranges above approximately 3.5 μm or even above approximately 4.3 μm.
Measuring can take place in a transmission arrangement, wherein measuring in transflection arrangement is also known. The radiation emitted by the laser or the diode laser is guided through the measurement medium, and following interaction with the measurement medium is detected by a suitable detector.
Gases can be detected that include at least one characteristic absorption band or absorption line in the wavelength range used. Gases that can be detected by laser absorption spectroscopy include, among others, oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), amines, ammonia (NH3), hydrogen sulphides (H2S), sulphur oxides (SO2), hydrogen halide compounds such as HCl or HF, gaseous water or even mixtures thereof.
For the determination of, for example, oxygen, a laser is suitable that, for example, emits or can be modulated in the region of 760 nm, while for the determination of NH3 a laser is suitable that emits or can be modulated in the region around 1500 nm, because in these regions there is in each case a strong absorption band of oxygen or NH3.
In this document, the terms “measurement environment” and “measurement medium” designate the environment or the medium in which measuring takes place, or which environment or medium is analysed.
U.S. Pat. No. 5,331,409 A discloses a gas analyser with a tunable diode laser and a connected gas analyser arrangement into which the measurement medium to be analysed is introduced, and which gas analyser arrangement includes several gas measurement cells. The radiation is directed by a collimator lens to a first measurement cell that includes a beam splitter, which directs the radiation through separate focusing lenses to a further measurement cell or a reference cell. The measurement cell and the reference cell each include a detector. However, the use of these lenses can give rise to interference that impedes the detection of the absorption bands. In order to suppress this effect to the greatest extent possible, in this arrangement the measurement cells can be designed to be particularly long.
With the use of a gas analyser in a process environment, such as in a container, the laser radiation can be coupled through a process window into the measurement medium or into the measurement environment. The process window can be a lens, as disclosed in U.S. Pat. No. 5,331,409 A. Furthermore, gas analysers are known that include wedge-shaped windows or normal windows as process windows. The wedge-shaped windows are frequently installed so as to be slightly oblique to the optical axis. Normal windows are predominantly installed below the Brewster angle; in other words also so as to be oblique to the optical axis. Depending on the alignment of the laser polarisation, installation of the process window below the Brewster angle can in itself result in signal loss, which even by the most precise adjustment possible cannot always be completely suppressed. Furthermore, with these solutions, frequently a pair of windows are installed in order to adequately correct the optical beam path, and furthermore, the process windows, for example, are coated with an anti-reflection coating in order to suppress interference effects that would otherwise occur. Interference effects can have a negative influence on the measuring results, and furthermore can result in temperature-dependent drift. In addition, because of the adjustments, precise installation of such windows can be time-consuming, and the manufacture of high-precision wedge-shaped windows can be expensive.
With the use of gas analysers in process facilities or measuring environments, above all in harsh environmental conditions, coated optical elements can, in certain circumstances, result in the coating being attacked and damaged or destroyed, which in turn results in a loss of measuring accuracy. In this context the term “harsh environmental conditions” refers to measuring environments with, for example, rather high or low temperatures, rather high or low pressures, and/or measuring environments where aggressive chemicals are used. In particular in the use in process facilities or in atmospheric research it is, furthermore, very difficult to ensure adequate adjustment of the process windows during the entire measurement cycle and/or during the service life of the gas analyser, because re-adjustments can only be made after deinstallation of the gas analyser.
Absorption-spectroscopic gas analysers can, for example, be used for in-situ monitoring of gas concentrations, in particular of oxygen, in potentially explosive processes or process facilities, for example in oil refineries or during combustion processes. Atmospheric research is another field of application.
For controlling and/or analysing such processes and measuring environments the gas analysers used should furnish measurement values at a high level of reproducibility with great reliability and with as little maintenance as possible.